Materials and technology issues for SiGe heterojunction bipolar transistors

Abstract

This paper reviews progress in SiGe heterojunction bipolar technology, with particular emphasis on the influence of materials issues on the technology performance. Low-temperature epitaxy and transient enhanced diffusion are identified as the main materials issues. Three epitaxy approaches are described, namely differential epitaxy, selective epitaxy, and combined selective and non-selective epitaxy. Differential epitaxy has the advantage of a simple growth process, selective epitaxy the advantage that an extrinsic base implant is not needed, and combined expitaxy the advantage that the Si collector and SiGe base can be grown in a single step. Transient enhanced diffusion of the boron in the base is shown to be a constraint on transistor performance, and the incorporation of a carbon concentration of ≈1×10 20 cm −3 in the base is shown to ease this constraint. Preliminary results on SiGe HBTs on wafer bonded SOI substrates are reported. The SOI transistors have characteristics comparable to those of bulk transistors, but the advantage of a gain that is a factor of approximately two higher.